The present invention relates to a voltage level display circuit and more particularly to a circuit for providing a display of the magnitude of a reference voltage input level to a comparator circuit.
Various instruments are used for measuring the voltage-time characteristics of an applied signal having a magnitude that varies as a function of time. Such devices measure the time displacement between the point of time at which the applied signal passes one reference voltage level and the point of time at which the applied signal pasess a second reference voltage level. These devices include a comparator with the applied signal coupled to one input and a reference voltage signal coupled to the second input. The comparator produces a transition in output state when the magnitude of the applied signal exceeds the magnitude of the reference voltage level.
Such devices typically include two channels, each having a comparator having an applied input signal and a reference voltage input signal. The comparator of the first channel provides a transition in output state when the applied signal passes the reference voltage level of the first comparator and the comparator of the second channel provides a second transition in output state when the applied signal passes the reference voltage level of the second comparator.
The output of the first comparator may be coupled to a suitable counter to initiate the counting of regularly recurring time pulses and the output of the second comparator may be coupled to the counter to terminate the counting of the pulses. The time displacements between the point of time at which the applied signal passes the first reference voltage level and the point of time at which the applied signal passes the second reference voltage level may be obtained by counting the pulses and multiplying the pulses by the period of each pulse.
Alternatively, a first applied signal may be coupled to the signal input of the comparator of the first channel and a second applied signal may be applied to the signal input of the second channel. The time period between the point of time at which the first applied signal passes the reference voltage level associated with the first channel and the point of time at which the second applied signal passes the reference voltage level associated with the second channel may be obtained by the pulse counting technique described above.
A microprocessor is used to provide a start digital signal corresponding to the desired reference voltage, which is coupled to a digital to analog converter in the first channel and a stop digital signal corresponding to the desired second reference voltage level is coupled to a digital to analog signal in the second channel. The start digital signal produced by the microprocessor is transmitted to a digital to analog converter in the first channel which converts the digital start signal to an analog reference voltage level and couples it to the reference voltage level input of the comparator of the first channel. The reference voltage level input to the comparator in the first channel is compared with the applied signal and the comparator provides a transition in output state when the applied signal passes the reference voltage level to thereby initiate a counting function.
The stop signal produced by the microprocessor is transmitted to a digital to analog converter in the second channel which converts the digital start signal to an analog signal and couples it to a reference voltage input level to the comparator of the second channel. When the applied signal coupled to the input of the comparator of the second channel passes the magnitude of the stop reference voltage level, the second comparator provides a transition in output state which is also coupled to the counter to terminate the counting function. Thus, the time displacement between the point of time at which the applied signal passes the start reference voltage level and the point of time at which the applied signal passes the stop reference voltage level may be measured by multiplying the number of pulses by the period of pulses.
In order to observe the magnitude of the start reference voltage level and the stop reference voltage level, various prior art devices have provided a digital voltmeter connected to the reference voltage level input of the comparator of each channel. The output of the digital voltmeter is applied to a display which displays the reference voltage input signal to each channel.
The requirement of a digital voltmeter for purposes of displaying the start reference voltage signal and the stop reference voltage signal increases the expense of the equipment substantially. Furthermore, it increases the complexity of the equipment and therefore the susceptibility of the device to error.